This invention relates to electrolytes which function as a source of alkali metal ions for providing ionic mobility and conductivity. The invention more particularly relates to electrolytic cells where such electrolytes function as an ionically conductive path between electrodes.
Electrolytes are an essential member of an electrolytic cell or battery. In one arrangement, a battery or cell comprises an intermediate separator element containing an electrolyte solution through which lithium ions from a source electrode material move between cell electrodes during the charge/discharge cycles of the cell. The invention is particularly useful for making such cells in which the ion source electrode is a lithium compound or other material capable of intercalating lithium ions, and where an electrode separator membrane comprises a polymeric matrix made ionically conductive by the incorporation of an organic solution of a dissociable lithium salt which provides ionic mobility.
Early Lithium Metal Cells
Early rechargeable lithium cells utilized lithium metal electrodes as the ion source in conjunction with positive electrodes comprising compounds capable of intercalating the lithium ions within their structure during discharge of the cell. Such cells relied, for the most part, on separator structures or membranes which physically contained a measure of fluid electrolyte, usually in the form of a solution of a lithium compound, and which also provided a means for preventing destructive contact between the electrodes of the cell. Sheets or membranes ranging from glass fiber, filter paper or cloth to microporous polyolefin film or nonwoven organic or inorganic fabric have been saturated with solutions of an inorganic lithium compound, such as LiClO4,LiPF6, or LiBF4, in an organic solvent to form such electrolyte/separator elements. The fluid electrolyte bridge thus established between the electrodes has effectively provided the necessary Li+ ion mobility at conductivities in the range of about 10xe2x88x923 S/cm.
Ion, Rocking Chair Cells and Polymer Cells
Lithium metal anodes cause dendrite formation during charging cycles which eventually leads to internal cell short-circuiting. Some success has been achieved in combatting this problem through the use of lithium-ion cells in which both electrodes comprise intercalation materials, such as lithiated metal oxide and carbon (U.S. Pat. No. 5,196,279), thereby eliminating the lithium metal which promotes the deleterious dendrite growth. Another approach to controlling the dendrite problem has been the use of continuous films or bodies of polymeric materials which provide little or no continuous free path of low viscosity fluid in which the lithium dendrite may propagate. These materials may comprise polymers, e.g., poly(alkene oxide), which are enhanced in ionic conductivity by the incorporation of a salt, typically a lithium salt such as LiClO4, LiPF6, or the like. A range of practical ionic conductivity, i.e., over about 10xe2x88x925 to 10xe2x88x923 S/cm, was only attainable with these polymer compositions at well above room temperature, however. (U.S. Pat. Nos. 5,009,970 and 5,041,346.)
xe2x80x9cSolidxe2x80x9d and xe2x80x9cLiquidxe2x80x9d Batteries of the Prior Art
More specifically, electrolytic cells containing an anode, a cathode, and a solid, solvent-containing electrolyte incorporating an inorganic ion salt were referred to as xe2x80x9csolid batteriesxe2x80x9d. (U.S. Pat. No. 5,411,820). These cells offer a number of advantages over electrolytic cells containing a liquid electrolyte (i.e., xe2x80x9cliquid batteriesxe2x80x9d) including improved safety factors. Despite their advantages, the manufacture of these solid batteries requires careful process control to minimize the formation of impurities. Solid batteries employ a solid electrolyte matrix interposed between a cathode and an anode. The inorganic matrix may be non-polymeric [e.g., xcex2-alumina, silver oxide, lithium iodide, etc.] or polymeric [e.g., inorganic (polyphosphazene)polymers] whereas the organic matrix is typically polymeric. Suitable organic polymeric matrices are well known in the art and are typically organic polymers obtained by polymerization of a suitable organic monomer as described, for example, in U.S. Pat. No. 4,908,283.
Examples of solvents known in the art are propylene carbonate, ethylene carbonate, xcex3-butyrolactone, tetrahydrofuran, glyme (dimethoxyethane), diglyme, tetraglyme, dimethylsulfoxide, dioxolane, sulfolane, diethoxyethane, and the like. These are examples of aprotic, polar solvents.
Heretofore, the solid, solvent-containing electrolyte has typically been formed by one of two methods. In one method, the solid matrix is first formed and then a requisite amount of this material is dissolved in a volatile solvent. Requisite amounts of the inorganic ion salt and the electrolyte solvent (usually a glyme and the organic carbonate) are then added to the solution. This solution is then placed on the surface of a suitable substrate (e.g., the surface of a cathode) and the volatile solvent is removed to provide for the solid electrolyte. In another method, a monomer or partial polymer of the polymeric matrix to be formed is combined with appropriate amounts of the inorganic ion salt and the solvent. This mixture is then placed on the surface of a suitable substrate (e.g., the surface of the cathode) and the monomer is polymerized or cured (or the partial polymer is then further polymerized or cured) by conventional techniques (heat, ultraviolet radiation, electron beams, etc.) so as to form the solid, solvent-containing electrolyte. When the solid electrolyte is formed on a cathodic surface, an anodic material can then be laminated onto the solid electrolyte to form a solid battery (i.e., an electrolytic cell).
More recently, a highly favored electrolyte/separator film is prepared from a copolymer of vinylidene fluoride and hexafluoropropylene. Methods for making such films for cell electrodes and electrolyte/separator layers are described in U.S. Pat. Nos. 5,418,091; 5,460,904; and 5,456,000 assigned to Bell Communications Research, each of which is incorporated herein by reference in its entirety. A flexible polymeric film useful as an interelectrode separator or electrolyte member in electrolytic devices, such as rechargeable batteries, comprises a copolymer of vinylidene fluoride with 2 to 25% hexafluoropropylene. The film may be cast or formed as a self-supporting layer retaining about 20% to 70% of a high-boiling solvent or solvent mixture comprising such solvents as ethylene carbonate or propylene carbonate. The film may be used in such form or after leaching of the retained solvent with a film-inert low-boiling solvent to provide a separator member into which a solution of electrolytic salt is subsequently imbibed to displace retained solvent or replace solvent previously leached from the polymeric matrix.
Electrolyte Breakdown
Regardless of which technique is used in preparing an electrolyte/separator, a recurring problem has been the loss of effectiveness of the electrolyte. The electrolyte has been observed to change color, evidencing a degradation that has not been well understood. There is presently no effective means to maintain the useful serviceability of the electrolyte.
In view of the above, it can be seen that it is desirable to have a novel, economical means for maintaining electrolyte integrity; and which maintains cell capacity in a variety of electrolyte/separator configurations, including those described above as exemplary.
The present invention provides an additive for an electrolyte solution of an electrochemical cell. The additive provides an electrolyte solution stabilized against decomposition during storage and during cyclic operation of an electrochemical cell. The additive is a dialkylamide, desirably a N,N-dialkylamide, and preferably is N,N-dimethylacetamide (DMAC). Advantageously, the additive prevents undesired decomposition of cell components, and particularly electrolyte solution components. Such undesired decomposition is evidenced by a change in color of the electrolyte solution, and may also result in undesired gaseous by-products. Gaseous by-products lead to volumetric expansion and possible rupture of the cell. The additive is usable with a variety of carbonaceous and metal oxide electrode active materials, providing improved performance without decomposition, which would otherwise occur, absent the additive.
In addition, the DMAC additive breaks down at potentials at or near an overcharge condition, nominally at or over about 4.4 volts. Thus, in a condition at or near cell overcharge condition, the DMAC additive aborbs excess charge energy by degrading at or just about 4.4 volts. This protects the active material from electrochemical damage by preventing the attainment of the damage threshold voltage, about 4.7 volts or higher, for lithium metal oxide active materials such as lithium manganese oxide, lithium cobalt oxide and lithium nickel oxide.
In one embodiment, the invention provides an electrochemical cell having an electrolyte which comprises a solute, a solvent, and the additive of the invention. The solute is a salt of lithium. The solvent comprises one or more aprotic, polar solvents. The dialkylamide of the invention is usable with a variety of solvents and salts. Exemplary solvents are carbonates; lactones; propionates; five member heterocyclic ring compounds; and organic solvent compounds having a low alkyl (1-4 carbon) group connected through an oxygen to a carbon, and comprising Cxe2x80x94Oxe2x80x94C bonds. Exemplary solvents are selected from the group consisting of propylene carbonate (PC), ethylene carbonate (EC), methyl ethyl carbonate (MEC), also referred to as ethyl methyl carbonate (EMC) diethyl carbonate (DEC), dipropyl carbonate (DPC), dimethyl carbonate (DMC), butylene carbonate (BC), dibutyl carbonate (DBC), and vinylene carbonate (VC). Among the preferred solvents are EC/DMC, EC/DEC, EC/DPC, and EC/EMC. With these combinations, there may also be used ethyl propionate (EP). Particularly preferred is EC/DMC/DMAC and EC/DMC/DMAC/EP. Any amount of DMAC added to the electrolyte solvent is helpful. Practical amounts are in the range of up to 20% by weight of the solvent mixture. The DMAC additive, from a practical point of view, may be present in the solvent mixture in an amount of 0.1% to 20% by weight of the solvent mixture. A range of 1% to 5% is preferred.
The DMAC enhances the thermal stability of lithium salts, such as LiPF6, and also enhances stability of co-solvents. Lithium salts are known to be subject to decomposition, and DMAC is useful to prevent such decomposition. As a result, the ionic species of the salt are preserved for ion transport. The additive of the present invention inhibits, prevents, or at least reduces and minimizes undesired side reaction which causes decomposition of cell components, and also prevents, inhibits, or reduces evolution of gaseous by-products which occur as a result of such decomposition. Advantageously, the additive of the present invention exhibits good performance and is compatible with a wide range of salts, solvents, and electrode active materials. Good performance is achieved even with carbonaceous electrode active material and with transition metal electrode active material which show poor performance when used in comparative conventional cells without the additive.
Objects, features, and advantages of the invention include an improved electrochemical cell or battery having improved charging and discharging characteristics; a large discharge capacity; and which maintains its integrity over a prolonged life cycle, as compared to presently used batteries and cells. Another object is to provide an electrolyte mixture which is stable with respect to electrode active materials, and which demonstrates high performance, and which does not readily decompose, evaporate, or solidify. It is also an objective of the present invention to provide cells with electrolyte solutions compatible with other cell components, avoiding problems with undesired reactivity, decomposition, and gas formation.
These and other objects, features, and advantages will become apparent from the following description of the preferred embodiments, claims, and accompanying drawings.